For slicing a silicon ingot, conventionally, a multi-wire saw has been used that is capable of slicing a large number of wafers at a time with a relatively small kerf loss. FIG. 1 shows a device constitution of a basic multi-wire saw for slicing a silicon ingot for producing a wafer for a solar cell. In a multi-wire saw 10 shown in the figure, numeral 1 denotes a silicon ingot, which is fixed by adhesion to a working plate 2, and in general, a material having a cross section of about 150 mm square and a length of about 400 mm is relatively frequently used.
Numeral 3 denotes a wire fed from a wire feeding mechanism 4, which is wound and suspended on two wire guide rollers 5 at a pitch of about from 0.3 to 0.4 mm and then wound up by a wire winding mechanism 6. A piano wire having a diameter of 0.16 mm is generally used as the wire 3. The wire 3 is fed at a speed of about 600 m/min while the feeding mechanism 4, the two wire guide rollers 5 and the winding mechanism 6 are driven with motors (which are not shown in the figure) that are synchronously controlled, and a prescribed tension is applied to the wire 3 by controlling the position of a tension roller 7. Slurry is applied to the wire 3 from a slurry agitating and feeding tank 8 through a slurry applying head 9. In this state, the ingot 1 fixed by adhesion to the working plate 2 is fed downward.
The slicing operation of the silicon ingot 1 using the wire saw 10 is effected in such a manner that a slicing slurry containing abrasive powder is fed between the running wire 3 and the ingot 1, and the abrasive powder are pressed onto the ingot 1 and simultaneously are rotationally moved with the wire 3, whereby microcracks are formed in the surface layer of the ingot 1 to scrape away the surface layer as silicon fine powder. In the slicing operation of the silicon ingot 1 in this manner, it is demanded to decrease the kerf loss and to decrease the thickness of the wafers in order to improve the yield of the wafers and to reduce the cost of materials of the wafers.
FIG. 2 is an enlarged view of the slicing part of the silicon ingot 1 using the multi-wire saw 10 and shows the relationship among the parameters upon slicing the silicon ingot 1. FIG. 3 is a schematic diagram showing the force applied to the wire in the slicing groove of the silicon ingot 1. In FIGS. 2 and 3, the following empirical expressions have been generally known in the art as showing the relationship among the feeding speed V of the silicon ingot 1, the feeding speed U of the wire 3, the slicing resistance P, the displacement δx of the wire 3 in the direction perpendicular to the slicing direction, the displacement δy of the wire 3 in the slicing direction, and the tension T of the wire 3.P∝V/U  (1)δx∝c P/T  (2)δy∝P/T  (3)
Upon feeding the slurry containing abrasive powder 11 to the interface to be sliced with the wire 3, the wire 3 is deflected to form the displacement δy, and the slicing resistance P is formed. The slicing resistance P is gradually increased but reaches constant at a prescribed value. At the slicing interface, the abrasive powder 11 are not uniformly dispersed, and the displacement δx of the wire 3 is formed by acting a force proportional to the slicing resistance P in the direction perpendicular to the slicing direction.
In the case where the value of δx is increased, wafers obtained by slicing the silicon ingot 1 suffer board warpage, unevenness in thickness and minute surface irregularities, so as to deteriorate the quality of the wafers. It may be suggested from the expression (2) that the slicing resistance P should be decreased in order to decrease δx.
Accordingly, the feeding speed V of the silicon ingot 1 may be decreased, or the feeding speed U of the wire 3 may be increased, as understood from the expression (1), but in the case where the feeding speed V of the ingot 1 is decreased, the slicing time is prolonged to deteriorate the production efficiency. In the case where the running speed U of the wire is increased, the consumption amount of the expensive wire is increased to increase the running cost for slicing. In order to decrease the kerf loss, it is necessary to decrease the diameter of the wire 3, but the breaking strength of the wire 3 is decreased thereby to provide necessity of decreasing the tension T applied to the wire 3. In the case where the tension T is decreased, the displacement δx of the wire 3 is increased as understood from the expression (2), and thus the wafers are deteriorated in quality as having been described above.
Microcracks remain in the surface layer of the wafers after slicing. Upon producing a solar battery cell by processing the wafers, it is necessary to remove firstly the damaged layer by etching, and thus the wafer thickness is further decreased after slicing the ingot. In the case where the wafer thickness becomes smaller, the damage rate of the wafers in the transporting and processing steps of the wafers is increased. The depth of the damaged layer is about 10 μm in the conventional techniques, which impairs reduction in thickness of the wafers.
As a method for removing the problem, fundamental studies have been made for a wire saw using slurry containing an alkaline aqueous solution and abrasive powder. It has been reported that in the case where a chemical dissolution function is imparted to a processing liquid upon slicing a silicon ingot, the resistance on moving the wire (which is hereinafter referred to as a wire pulling resistance) is decreased, and the crack depth on the surface layer of the ingot is decreased (see, for example, in Non-patent Document 1).
FIG. 4 is a conceptual diagram for describing the effect obtained by imparting a chemical function to the slurry. In the figure, symbol A1 denotes the state where a neutral slurry is used, and A2 denotes the relationship between the time and the wire pulling resistance F of the wire 3 in this case. Symbol B1 denotes the state where alkaline slurry is used, and B2 denotes the relationship between the time and the wire pulling resistance F of the wire 3 in this case.
The abrasive powder 11 in the slurry are pressed on the ingot 1 and rotationally moved with the wire 3, whereby cracks 12 formed one after another on the surface of the ingot 1 are connected to each other, and fine powder of silicon is formed at regions where they reach the surface of the ingot 1 and discharged from the slicing groove with the abrasive powder rotationally moved and the flowing slurry liquid. The formation of fine powder means elimination of cracks. It has been studied therein that in the case where alkaline slurry is used, the cycle time from formation to elimination of cracks is shortened as understood from the comparison between A2 and B2, and fine powder is formed on and discharged from the surface of the ingot 1 before application of a large removing power (wire pulling resistance F) at the slicing interface.
In FIG. 2, the tension T is applied to the wire 3, whereby the displacement δx of the wire 3 in the direction perpendicular to the slicing direction is to be a prescribed value, but the wire pulling resistance F is added thereto in the portion of the wire 3 on the side where the wire is withdrawn from the ingot 1. In the case where the wire pulling resistance F is decreased, the force applied to the wire 3 is decreased. The diameter of the wire 3 is decreased through movement of the wire 3 from the feeding side to the winding side of the wire since the wire is continuously abraded with the abrasive powder. In the case where the wire pulling resistance F is decreased, the abrading force is also decreased to suppress reduction in diameter of the wire 3. The reduction of the wire pulling resistance F applied to the wire 3 and the reduction of the abrading amount of the wire provide capability of reducing the diameter of the wire, and thus the kerf loss can be decreased.
On the line of the aforementioned approach, such a single wire saw has been proposed that uses an alkaline slurry of pH 9 or more at from 30 to 80° C. or a acidic slurry of pH of from 3 to 6 at from 25 to 65° C. (see Patent Document 1).
Such a method of slicing a material to be processed has been proposed in that an etching liquid containing no abrasive particle is coated on a wire, and the temperature of the etching liquid is increased to 50 to 60° C. by frictional heat formed between the wire and the material to be processed (see Patent Document 2).
Non-Patent Document 1:
Electronics yo Kessho Zairyo no Seimitsu Kako (Precision Processing of Crystalline Materials for Electronics), published on January 30, Showa 60 (1980), by Science Forum Inc.
Patent Document 1: JP-A-2-262955
Patent Document 2: JP-A-2-298280